Usually rotovators can just break up the roots and make the whole thing worse. I would cut down the unwanted growth and then get some old carpet and cover the part of the area you want to clear. You say wildlife garden - the wildlife will love the cow parsley and the hogwort.

I have to say it doesn't look good Picksey. THe green on the stem is algae and nothing to do with frost. Any sign of new leaves coming through. Look at the buds carefully and see how healthy they look.